1. The old Gremlins are good for fighting neutrals to "resurrect" golems, i.e., to reduce losses; the new ones are mainly useful during sieges to destroy catapults or towers.
2. I was very pleased with the new gargoyles; they can really harass the enemy, as they significantly increase the damage to all adjacent units. But if your hero doesn't specialize in Chaos magic, they might do more harm than good. In that case, Obsidian gargoyles are the better choice.
3. The situation with golems is very ambiguous. The choice largely depends on your play style, your opponent, and other factors. But subjectively, the Obsidian golems are slightly better. Although when I played with them, I regretted the lack of retaliatory strikes several times.
4. Again, the mages are ambiguous. But the White mages are very good at defending their own. You can even sacrifice the Fireball spell here, and reducing the cost of spells is not that important for a mage hero.
5. A random spell is still more useful than increasing/decreasing luck by 1. If it were 2, I would seriously consider the choice.
6. Retaliation is still a very useful ability, especially for a monster like Rakshasa. I choose them, although I don't deny that Kshatriyas are cool.
7. Here, I often think about what will be more useful in the next battle: Lightning Bolt or Cloud. I think Lightning Bolt is for sieges, and Cloud is for everything else.